German patent specification DE 44 36 254 C1 discloses current transformers used to detect currents at the ends of a section of an electrical power supply system which is to be monitored with regard to the occurrence of an internal fault. In the known method, the currents obtained by means of the current transformers are converted, in a measured value preprocessing device, into root-mean-square-value-proportional measurement quantities, with which differential and stabilization current values are obtained. In order to detect a fault on the section of a power supply system that is to be monitored, differential current values are monitored with regard to exceeding a predetermined lower limit value of the differential current (differential current limit value) and with regard to exceeding stabilization current values weighted with a characteristic curve factor. The trigger signal is generated if positive results of the two instances of monitoring are present simultaneously.
In the known method, special precautions have to be taken to guard against incorrect triggering on account of saturation phenomena in the current transformers. This is because, under certain circumstances, current transformers transform the measured values completely satisfactorily only for in each case a limited short time span of each period, because they enter into saturation in the case of relatively large current values. As a result of the saturation phenomena in the current transformers, intrinsically external faults with regard to the section to be monitored may mistakenly be classified as internal faults, which can then lead to undesirable triggering. In order to prevent this, care is taken to ensure that the outputting of a trigger signal is blocked after an external fault has been ascertained in the state of unsaturated current transformers. In this case, the blocking is not performed for a fixedly predetermined time, but rather is effected for a predetermined time duration starting from an instant which depends on the respective conditions. After this time duration has elapsed, the known method can then respond again to an internal fault.